In this paper, a combination of particle swarm optimization and genetic algorithm is used to perform layered design and optimization of a bilayer metalens across a wide bandwidth, which achieves the enhancement of the focusing and achromatic performance of the bilayer composite nanostructure metalens. Firstly, the optical modulation characteristics of the micro cell structure required for the concave-convex metalens are studied, and the phase database is constructed by scanning the radius of the micro cell structure at a fixed height by numerical simulation software Lumerical. After that, according to the phase profile of the bilayer metalens, the optimal radius-phase matrix is found in the phase database, that is, the optimal element structure corresponds to each position of each layer of metalens. Finally, the bilayer metalens is constructed layer by layer, according to the optimization results, and the simulation experiments verify that the bilayer metalens can achieve the focusing in the near-infrared band (1000 nm, 1250 nm), and the achromatic effect is better than that of the single-layer metalens at the same focal length. This metalens provides a solution for the design of bilayer composite nanostructured metalens in the near-infrared band.